Mass spectrometer leak detector and ion source therefor having magnetic focusing means



Oct. 4, 1966 w. E. BRIGGS 3,277,295

MASS SPECTROMETER LEAK DETECTOR AND ION SOURCE THEREFOR HAVING MAGNETICFOCUSING MEANS Filed April 7, 1966 5 Sheets-Sheet 1 FlG.l

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DIFFUSION PUMP 3,277,295 HEREFOR Oct. 4, 1966 w. E. BRIGGS METER LEAKDETECTOR AND ION SOURCE T HAVING MAGNETIC FOCUSING MEANS MASS SPECTRO 5Sheets-Sheet 2 Filed April 7, 1966 Oct. 4, 1966 w. E. BRIGGS 3,

MASS SPECTROMETER LEAK DETECTOR AND ION SOURCE THEREFOR HAVING MAGNETICFOG-USING MEANS 5 Sheets-Sheet 3 Filed April 7, 1966 FIG. 4A

FIG.4B i

Oct. 4, 1966 w. E. BRIGGS 3,

MASS SPECTROMETER LEAK DETECTOR AND ION SOURCE THEREFOR HAVING MAGNETICFOGUSING MEANS 5 Sheets-Sheet 4 Filed April 7, 1966 FIG. 5

Oct. 4, 1966 w. E. BRIGGS 3,277,295

MASS SPECTROMETER LEAK DETECTOR AND ION SOURCE THEREFOR HAVING MAGNETICFOCUSING MEANS Filed April 7, 1966 5 Sheets-Sheet 5 PHILIPS |ON GAUGE H1SOURCE E6 29? 2 R 252 REGION HA ANALYZER REGION H r \7 296 250 COLLECTORREGION United States Patent 3,277,295 MASS PETROMETER LEAK DETECTOR ANDIUN SOURQE THEREFOR HAVING MAGNETIC FOCUSING MEANS Walton E. Briggs,Lynnfield, Mass., assignor to National Research Corporation, Newton,Mass., a corporation of Massachusetts Filed Apr. 7, 1966, Ser. No.540,906 20 Claims. (Cl. 250-419) This application is acontinuationdn-part of application Serial No. 332,154 filed Dec. 20,1963 and now Patent No. 3,265,890.

The present invention relates to an improved construction of vacuumapparatus and more particularly to an improved construction of a massspectrometer leak detector.

Mass spectrometer leak detection has been a familiar technique in thevacuum art since first practiced at the University of Minnesota in 1943.A mass spectrometer is set to detect a probe gas, such as helium, andthe probe gas is applied to the main vacuum system under investigation.The mass spectrometer samples gas from the pumping line of the systemunder investigation. A separate pumping system is provided for the massspectrometer and a throttle valve couples the mass spectrometer to themain vacuum system.

It is a principal object of the invention to provide an improved massspectrometer leak detector of simple construction with adequatesensitivity for its leak detection function.

In general, this object is accomplished by providing a thin, vacuumtight leak detector member with an external magnet. The member andmagnet are of simple construction. The leak detector electrodes aresupported in the member .by vacuum feedthroughs. Magnetic focusing isused in the ionization portion of the leak detector. The ion sourceelectrodes are made in the form of a disposable construction. Thealignment of the electrodes with the member is non-critical. Adequatesensitivity is achieved by adjusting the magnetic field outside thevacuum and by the ion source construction described below.

A further object of the invention is to provide a mass spectrometer leakdetector member, with internal Phillips gauge, of compact constructionwith easy access to all parts which must be serviced.

The member preferably has the form of a thin box with opposed parallelfaces and narrow side walls. The magnet comprises a yoke and pole platesstraddling the faces. Pairs of pole extensions extend inwardly from thepole plates to concentrate magnetic fields in the gauge, ion source andanalyzer portions of the member. Demountable vacuum feedthroughs supporta Phillips gauge, an ion source and a collector in an internal cavity ofthe body. The feedthroughs are in a side wall and in close proximity toeach other.

The magnet can be assembled with the member and placed in a leakdetector cabinet with the feedthroughs being accessible for ease ofservicing. Then the magnetic field is adjusted during operation of theleak detector to establish a desired sensitivity of the ion source.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises an improved machine possessing theconstruction and arrangement of parts exemplified in the followingdetailed disclosure and the accompanying drawings and the scope ofapplication of which will be indicated in the claims.

FIG. 1 is a sectional view showing the arrangement of spectrometer partsin the body member according to a preferred embodiment of the invention;

FIG. 2 is a sectional view transverse to FIG. 1 taken along the sectionIIII as indicated in FIG. 1;

FIG. 3 shows the construction of the Phillips gauge cathode liner;

FIG. 4 is a schematic diagram, in the form of an exploded section takenalong the line IVIV in FIG. 2, of the ion source and magnet polesaccording to a first and preferred embodiment of the inventionillustrating the general mechanical arrangement of parts and alsoindicating the electrical arrangement of parts in the ion source;

FIG. 4A is an isometric view of the ion source, per se;

FIG. 4B is a schematic diagram, similar in form to FIG. 4, illustratinga second embodiment of the invention;

FIG. 5 is an isometric, exploded view of the body member and magnetassemblies; and

FIG. 6 is an isometric view of a body member of the apparatus accordingto a third embodiment of the invention;

FIG. 7 is an isometric view of a magnet member of the apparatusaccording to a fourth embodiment of the invention; and

FIG. 8 is a partially sectional view of magnet pole plates and ionsource pole extensions according to a fifth embodiment of the invention.

FIGS. 1-5

The leak detector vacuum system is shown in black box form since suchsystems are well known in the art. Gas is sampled from the foreline of avacuum system under investigation to a leak detector pumping systemwhich includes a liquid nitrogen cold trap, a small diffusion pump (2inches nominal diameter) and a small forepump. A proportion of sampledgas is introduced to the mass spectrometer body member by using anadjustable throttle valve T to limit the diffusion pump.

The body member comprises a solid block 10. Crossing holes are machinedthrough the block to define an internal cavity 12 which serves as thegas flow path of the spectrometer. The cavity is enclosed by parallel,opposed planar faces 14 spanned by narrow side Walls 16. A gas flowopening 18 extends through a side Wall to the cavity. Auxiliary openings30, 50 and 70 also extend through aside wall to the cavity.

IONIZATION GAUGE (FIGS. 1, 2, 3)

A Phillips gauge is supported on a demountable vacuum closure 32 forinsertion and removal through opening 30. The Phillips gauge comprises apair of semi-cylindrical pole pieces 34 for concentrating a magneticfield within the gauge portion of the cavity, a disposable cathode 36which comprises a sheet bent to form two cathode faces 37 and a wireanode 38. The cathode 36 is sufiiciently resilient to bear against itssupporting magnet pole pieces 34. The anode and pole pieces are mountedin the closure 32. The body member 10, and consequently the cathode 36,are at ground potential. The anode 38 is maintained at a positivepotential of about 2000 volts.

ION SOURCE (FIGS. 1, 2, 4, 4A, 4B)

An ion source is supported on a demountable vacuum closure 52 forinsertion and removal through opening 50. The closure 52 comprises aconventional eight-pin vacuum tube feedthrough. The ion source comprisesan ion chamber electrode 54 which is open to the flow of gas from theinlet opening 18 and the Phillips gauge. The ion chamber electrode hasan ion exit slit 56 and upper and lower slits 58 for the admission ofelectrons. Upper and lower thermionic filaments 60 provide alternativesources of electrons for ionizing gas molecules in the ion chamberelectrode region. One of the filaments is operated and the other isreserved as a spare. When the first a filament burns out, the spare canthen be switched on until it is convenient to break the vacuum andreplace the entire ion source. The symmetrical alignment insures similaroperating characteristics with either filament. Positively biased ionrepeller electrode 62 is disposed in the ion chamber and opposite exitslit 56 to force a beam of ions out through the exit slit.

Focusing of the electrons is accomplished by a magnetic field disposedparallel to the aligned filaments 60. The space charge of the confinedelectron beam, in turn, aids in the focusing of the ion beam. The ionbeam is further focussed by cooperation of the ion repeller and ionchamber and by focus plates 64 disposed outside the ion chamber and inproximity to the exit slit 56.

Alignment of the ion source electrodes and construction of the ionsource emphasizes ruggedness and economy. The ion source is designed asa throwaway unit to be replaced when the filaments burn out or excessiveinsulating deposits build up on the electrodes. In the prior art,magnetic focus ion sources have relatively expensive and careful designof the ion chamber and repeller and focus electrodes to assure properalignment with an electrostatic or magnetic focus field. The filamentmust be provided in a replaceable form. The source construction and theadjustable magnetic field, described below, give sufficient sensitivityto the present ion source to allow a more rugged and inexpensiveconstruction.

The electrodes are all spot welded to the pins 1-8. The filaments aresupported on pins 1, 8, 4 and 5. The ion chamber electrode and repellerelectrode are supported between the filaments by pins 3 and 6,respectively. Pins 2 and 7 are longer than the other pins and areutilized to support focus plates 64 beyond exit slit 56. The relativeDC. bias imposed on the respective electrodes is indicated by theschematic drawing of a voltage divider in FIG. 4.

The series arrangement of the Phillips gauge between the gas flow inlet18 and the ion source substantially prevents filament contamination.This feature is disclosed and claimed in the above-cited parentapplication S.N. 332,154. Some contaminant gasses get past the Phillipsgauge, but these are not sufiicient to substantially reduce thesensitivity of or embrittle the hot filament(s) of the ion source. Ifdesired, an optical bafile may be placed in the cavity 12 between thegauge and ion source. Such a bafi le serves as a supplementarycollection surface for pumped gas particles (in addition to the gaugeelectrode structure).

ANALYZER (FIGS. 1, 5)

The ion beam is segregated by mass/charge ratios in a 90 analyzersection 99 having an inlet port 92 and exit port 94. A ground slit plate66 is disposed between the ion source and analyzer. A ceramic locatingblock 68 is secured in the cavity to locate the removable ion sourcewith respect to the analyzer and the magnetic field. The fit between pin7 of the ion source and locator 68 is a loose one, simply sufficient toobtain alignment of exit slit 56 of the ion source with the axes ofground plate 66 and port 92 within a few degrees tolerance.

The member 10 has its narrowest width in the region 90 to provide aminimum magnet gap for the external magnet. Face portions 50 of magneticmaterial are welded into position to concentrate a high field, on theorder of 2000 gauss, through the analyzer portion.

COLLECTOR (FIG. 1)

Conventional collector electrodes and preamplifier circuitry aresupported on a demountable vacuum closure 72 for insertion and removalthrough opening 70. The electrodes comprise a suppressor 74, aground-plate 76, and a collector plate 78, all supported in a collectortube 80 with an entrance port 82. A preamplifier tube 84 and its gridresistor 86 are supported in the cavity 12 by closure 72.

t MAGNET (FIGS. 1, 2, 5

The mass spectrometer mechanical assembly is shown in exploded form inFIG. 5. The magnet 10%) comprises a yoke 102 and a pair of pole plates104. Pairs of pole extensions 106, 108 and 110 extend inwardly from theplates to provide the magnetic fields for the analyzer (H Phillips gauge(H and ion source (H re spectively. Knurled handles 112 are provided forrotating the magnet pole extensions 110 to vary the sensitivity of theion source.

The yoke 102 is made of alnico while the pole plates and pole extensionsare of soft iron. This simple form of the magnet is made possible by theabove-described narrow construction of the body, particularly of theanalyzer portion 80.

A clamping bar 114 secures the magnet to the body 10. A conventionalvacuum flange 116 and pipe 118 can be provided as part of inlet 18 toprovide a demountable connection to the pumping system.

ACCESSIBILITY (FIGS. 1, 2, 5)

The assembled mass spectrometer leak detector may be connected to a coldtrap (not shown) of a conventional leak detector pumping and sampleinlet system (such as that described by Thomas, Williams and Hipple inReview of Scientific Instruments, vol. 17, p. 368). The massspectrometer and cold trap are recessed in a cabinet. The operator canreach down into the cabinet to turn knurled handles 112 to adjust theion source sensitivity while the leak detector is operating or toreplace the electrodes mounted on any of closures 32, 52, 72, withoutdisturbing magnet 100. It would, of course, be necessary to break vacuumbefore removing any of the closures.

Certain changes can be made in the above-described apparatus withoutdeparting from the scope of the invention herein involved. For instance,the cammed pole pieces can be of other constructions which provide anadjustable magnetic field through the ion source without disturbing theanalyzer and Phillips gauge magnet fields. Many of the structuralconcepts described above are of general utility in ion sources otherthan those used in leak detectors.

FIG. 4B

A second embodiment of the invention is shown in FIG. 4B which shows amodified ion source. The ion source of this embodiment differs from thatof the above-preferred embodiment in that the ion chamber electrode 454is a cylinder and the ion repeller 462 is a disk compared to theU-shaped ion chamber 54 and U-shaped repeller 62 of the preferredembodiment (shown in FIGS. 2, 4 and 4A).

The FIG. 4B embodiment can also accommodate two filaments 58 either ofwhich can be activated to send electrons through one of the opposedopenings 58 in the ion chamber electrode 454.

FIG. 6

Another embodiment of the invention is shown in FIG. 6. In thisembodiment, the body member is formed of brazed-together tubes 296, 297and 298 which provide the Phillips gauge, ion source and collectorregions. These tubes may be interconnected by additional conduits suchas tube 299. A bridging section 205 with magnetic wall inserts 250 maybe used to form a narrow analyzer. Phillips gauge, ion source, andcollector are mounted on demountable closures 232, 252, and 272,respectively (similar to the closures 32, 52 and 72 of the preferredembodiment above). The electrode and magnet structure of this embodimentmay be the same as that of any of the other embodiments herein.

Magnetic fields are provided as indicated by the arrows H H and H forthe gauge, ion source, and analyzer, respectively.

The brazed tube construction of the FIG. 6 embodiment is in contrast tothe machined block construction of the FIG. 1 embodiment. But bothembodiments have it in common that a mass spectrometer body member withopposed faces and relatively narrow side walls is straddled by magnetpoles, the body member having demountable closures on the side wallsupporting electrodes within the mass spectrometer body. The FIG. 1embodiment has the three demountable closures 32, 52, 72 in closeproximity to each other and it will be understood that the FIG. 6embodiment could be modified by rearranging tube 298 so that closure 272is adjacent closure 252.

FIG. 7

Another embodiment of the invention is shown in FIG. 7. A common magnet202 is provided for the analyzer and ion source (with a separate magnetbeing provided for the gauge). A narrow analyzer gap is providedadjacent the yoke. Pole plate extensions 204 provide a link to polepieces 110 which provide the ion source field. The pole pieces 110 arethe same as in FIGS. 2 and 5 and include handles 112. The analyzer gapis narrower than the ion source gap.

The magnet of FIG. 7 may be used with the machined body member of FIGS.1-5 or the brazed tube body member of FIG. 6.

The pole plates 204- can be enlarged, if desired to accornmodatePhillips gauge pole pieces.

FIG. 8

Another embodiment of the invention is shown in FIG. 8 wherein amodification of the pole plates for the adjustable ion source field (His shown. The magnet structure may be generally as shown in FIG. 5, orin FIG. 7, and it includes pole plates 304. Each of the pole plates hasa hole 305 in the desired region of the ion source. Pole pieces aremovable within the holes 305 along the edges thereof to provideadjustment of the direction of the field H The pole pieces may besecured in their various adjusted positions by washers (larger thanholes 305) and tie down screws or by other adjustable means.

Unlike the previous embodiments the FIG. 8 embodiment does not have theion source pole pieces extending inwardly. This is not as efiicient asthe other embodiments, but is acceptable, so long as the general fieldstrength between pole plates 304 is adequate for the ion source. Highsensitivity of the ion source is achieved by the spectrometer operatorwho moves pole pieces 310 from time to time.

There are various other modifications which can be made from thepreferred embodiment, which may be more or less desirable depending onthe particular requirements of the user. The common link of all theabove-disclosed embodiments, and equivalents thereof, is that aneconomical ion source (e.g., the ion source of FIGS. 4-4A or the ionsource of FIG. 4B) is provided and made comparable in performance to theexpensive ion sources of the prior art by the combination therewith ofexternal (outside the mass spectrometer body) magnet means (e.g., themagnet of FIG. 5; the magnet of FIG. 7; either of FIG. 5 or FIG. 7 withthe modification of FIG. 8), which are adjustable to achieve highsensitivity despite the small misalignments inherent in the simpleconstruction of the ion source. The general economy of the constructionis furthered by combining the adjustable magnet of the ion source andthe fixed analyzer magnet in a single structure (e.g. as in FIG. 5 orFIG. 7). Preferably, the fixed Phillips gauge magnet is combined in thesame structure as in FIG. 5.

An additional contribution to the art, interdependent with theforegoing, is presented in the subcombination of the ion source (FIGS.4-4A species or FIG. 4B species). This ion source accommodates a spacefilament, is quite economical and has utility in devices other 6 thanmass spectrometers, such as total pressure vacuum gauges.

An additional and discrete contribution to the art is presented in thegas flow series arrangement of the Phillips gauge and ion source which,surprisingly, essentially eliminates the long standing problem offilament contamination which has plagued the mass spectrometer. Thiscontribution is separately claimed in my above-cited parent application.

The contributions of the present invention are capable of being renderedin several different physical forms and contexts as shown here or inother forms not shown here. Therefore, it is intended that the abovedescription and accompanying drawings shall be interpreted asillustrative and not in a limiting sense.

What is claimed is:

1. An improved mass spectrometer comprising in combination:

(a) a body member having opposed faces and narrow side walls enclosing agas evacuated internal cavity, a gas flow inlet in the surface of thebody leading to the cavity and adapted to be connected to a system to beinvestigate-d, auxiliary openings in the surface of the body alsoleading to the cavity,

(b) demountable vacuum closure means covering said auxiliary openingsand supporting a cold cathode discharge gauge, ion source electrodes anda collector in said cavity,

(c) magnet means providing magnetic fields through the gauge, throughthe ion source, and through a portion of the cavity between the ionsource and collector, and comprising opposed pole plates external 01":the gauge and straddling the faces of the body with pairs of poleextensions extending inwardly from the plates to provide said fields andwherein the pole extensions which provide said ion source field areadjustable to vary the sensitivity of the ion source.

2. The mass spectrometer of claim 1 wherein said auxiliary openings arein the side walls in close proximity to each other to provide a commonregion of access to the mass spectrometer parts.

3. An improved construction of ion source for use in mass spectrometersand the like and comprising, in combination:

(a) a body member enclosing a cavity, a first opening in the surface ofthe body connected to the cavity and constructed and arranged as a gasflow inlet, a second opening in the surface of the body connected to thecavity, a demountable closure, said demountable closure covering saidsecond opening;

(b) an ion source structure mounted on said demountable closure andsuspended therefrom within the cavity and comprising ion chamberelectrode means and at least one filament, an ion repeller electrodedisposed adjacent the ion chamber, the ion repeller electrode and ionchamber electrode being constructed and arranged to provide asubstantially enclosed ionization volume with at least a first inletopening and a second exit opening, the filament being .arranged outsidesaid volume for injecting electrons into the opening through said inletopening, the repeller electrode being arranged opposite to said exitopening, at least one focus electrode disposed outside the ionizationvolume and in proximity to the exit opening, said filament, ion chamber,repeller and focus electrodes being individually supported from saidclosure and being constructed and arranged for insertion and removal ofsaid electrodes as a group through said sec-0nd opening;

(c) a magnet mounted adjacent the body with a pair of pole extensionsstraddling the portion of the body having the ion source and means foradjusting the pole extensions with respect to the magnet to adjust thedirection and position of the magnetic field axis to vary thesensitivity of the ion source and (d) the ion chamber being constructedand arranged so that the filament and ionization volume inlet openingare in a line essentially parallel to the direction of the magneticfield.

4. The apparatus of claim 3 wherein the said magnet pole extensions arerotatably mounted for pivoting about axes and are arranged as cams withrespect to said axes.

5. The apparatus of claim 3 wherein the extensions are cylindrical andtheir pivotal axes are offset from their cylindrical axes.

6. The apparatus of claim 3 wherein the magnet comprises parallel poleplates straddling the body with holes in the plates, the pole extensionsbeing mounted on said holes.

7. The apparatus of claim 6 wherein the pole extensions extend inwardlyfrom the plates toward the ion source to narrow the magnet gap.

8. The apparatus of claim 6 wherein the pole extensions are essentiallyflush with the plates along the inner faces of the plates.

9. The apparatus of claim 3 wherein the said ion chamber electrode meanscomprises a single electrode having a U-shape in a cross-section takensubstantially parallel to the direction of the magnetic field andwherein the repeller electrode has a U-sha-pe in a cross-section takensubstantially perpendicular to the magnetic field, the ion chamber andrepeller electrodes being cooperatively arranged to form a substantiallyenclosed volume therebetween.

10. The apparatus of claim 9 wherein the ion source has two filamentsaligned so that a line through the two filaments is essentially parallelwith the direction of the magnetic field.

11. The apparatus of claim 3 wherein two filaments are provided outsidethe ionization volume and wherein two inlet openings are provided in themeans forming said ionization volume, the openings and filaments beingessentially on a line parallel with the direction of the magnetic field.

12. The apparatus of claim 11 wherein the said ion chamber electrodecomprises a single electrode having a U-shape in a cross-section takensubstantially parallel to the direction of the magnetic field andwherein the repeller electrode has a U-shape in a cross section takensubstantially transverse to the magnetic field, the legs of the ionchamber U-shaped constituting opposed walls of the ion chamber electrodeand containing the said two openings of the ionization volume.

13. In a disposable ion source structure for operation in a magneticfield in mass spectrometers and the like, a demountable closure for avacuum system having mounted therein, via lead pins, an ion chamberelectrode, a repeller electrode, at least one focus electrode and atleast one thermionic filament, the improvement wherein said ion chamberelectrode consists of a U-shaped member and said repeller electrodeconsists of a U-shape member, the ion chamber and repeller electrodesbeing arranged transversely and in proximity to each other to form apartially enclosed ionization region, an ion exit aperture in the U-baseof the ion chamber electrode.

14. An improved mass spectrometer comprising, in combination:

(a) a body member having opposed faces and narrow side walls enclosing agas evacuated internal cavity, a gas flow inlet in the surface of thebody leading to O o the cavity and adapted to be connected to a systemto be investigated, at least first and second auxiliary openings in theside walls of the body also leading to the cavity,

(b) first and second demountable vacuum closures covering said auxiliaryopenings, one of said closures supporting ion source electrodes in saidcavity and the other closure supporting a collector in said cavity in aposition spaced away from said ion source electrodes,

(c) magnet means providing magnetic fields through the ion source andthrough an analyzer portion of the cavity between the ion source and thecollector, said magnet means comprising opposed plates external of thebody and straddling the faces of the body in close proximity theretowith pairs of pole extensions extending inwardly from the plate toprovide said fields, the pole extensions which provide said ion sourcefield being adjustable with respect to each other and with respect tothe body to vary the sensitivity of the ion source, the portions of thebody member face walls covering the analyzer portion being made ofmagnetic material to provide a short analyzer magnetic gap.

15. In a mass spectrometer comprising an ion source, analyzer andcollector means, an improved construction thereof comprising:

(a) a body member enclosing a cavity,

(b) ion source and collector means mounted in said cavity;

(c) common magnet means providing magnetic fields for the portion of thecavity containing the ion source electrodes and for a separate analyzerportion of the cavity between the ion source and collector means;

(c) said magnet means comprising a pair of poles external of the bodymember for providing said ion source field and means for adjusting saidpoles to adjust the direction and position of the ion source magneticfield without disturbing the analyzer mag netic field;

whereby structural misalignments of the ion source magnetic field arecompensated by adjustment of the ion source magnetic field to providehigh sensitivity mass analysis of low mass number gas samples,consistent with low cost construction of the ion source means.

16. The apparatus of claim 1 wherein the body member (a) comprises ablock with intersecting bores forming said cavity.

17. The apparatus of claim 1 wherein the body member (a) comprises aplurality of brazed tubes with the interiors of the tubes forming saidcavity.

18. The apparatus of claim 15 wherein the ion source means comprisesmeans forming an ionization volume having an inlet opening and afilament external of the volume, the filament and inlet opening beingaligned essentially parallel with the direction of the magnetic field.

19. The apparatus of claim 18 wherein the ionization volume issubstantially enclosed by a box-like electrode construction.

20. The apparatus of claim 18 wherein the ionization volume issubstantially enclosed by a cylindrical electrode construction.

No references cited.

RALPH G. NILSON, Primary Examiner.

ANTHONY L. BIRCH. Examiner.

1. AN IMPROVED MASS SPECTROMETER COMPRISING IN COMBINATION: (A) A BODYMEMBER HAVING OPPOSED FACES AND NARROW SIDE WALLS ENCLOSING A GASEVACUATED INTERNAL CAVITY, A GAS FLOW INLET IN THE SURFACE OF THE BODYLEADING TO THE CAVITY AND ADAPTED TO BE CONNECTED TO A SYSTEM TO BEINVESTIGATED, AUXILIARY OPENINGS IN THE SURFACE OF THE BODY ALSO LEADINGTO THE CAVITY, (B) DEMOUNTABLE VACUUM CLOSURE MEANS COVERING SAIDAUXILIARY OPENINGS AND SUPPORTING A COLD CATHODE DISCHARGE GUAGE, IONSOURCE ELECTRODES AND A COLLECTOR IN SAID CAVITY, (C) MAGNET MEANSPROVIDING MAGNETIC FIELDS THROUGH THE GUAGE, THROUGH THE ION SOURCE, ANDTHROUGH A PORTION OF THE CAVITY BETWEEN THE IRON SOURCE AND COLLECTOR,AND COMPRISING OPPOSED POLE PLATES EXTERNAL OF THE GUAGE AND STRADDLINGTHE FACES OF THE BODY WITH PAIRS OF POLE EXTENSIONS EXTENDING INWARDLYFROM THE PLATES TO PROVIDE SAID FIELDS AND WHEREIN THE POLE EXTENSIONWHICH PROVIDE SAID ION SOURCE FIELD ARE ADJUSTABLE TO VARY THESENSITIVITY OF THE ION SOURCE.